Electrical meters have long been used to measure energy consumption at a premises of a utility service consumer. Such meters are available in many forms, and have changed significantly over time. Traditional electrical meters were typically implemented as induction meters, which included pot coils that measure a voltage of a circuit to be metered, as well as current coils in series with the service to be monitored. Such coils could in turn induce movement of disks which are calibrated to rotate at a rate based on observed voltage, current and their phase relationship. Dials calibrated using such disk rotations could then be periodically monitored by a utility to determine consumption by the customer. Hence, by monitoring the voltage, current and their phase relationship delivered to a consumer, overall energy consumption and power could be determined.
Electrical service that is delivered to a consumer premises, and in particular to a commercial location, may have a current delivery capability as high as 5000 amps. Such service is not typically capable of being handled by a meter. As such, this electrical service was traditionally stepped down using instrument transformers to signal levels manageable by a meter.
Electrical meters as are discussed above were traditionally electromechanical, in that the voltage and current phase relationship generate magnetic fields proportional to the wattage, which acts on an aluminum disk within the meter that moves (typically rotates) proportionally to real power delivered to the premises.
More recently, most of the electromechanical meters as discussed above have been transitioned (at least for new installations) to meters including solid state measurement elements, which are capable of operating across wide voltage ranges. However, such meters were designed to be backwards compatible with prior metering technologies to allow them to be installed directly in place of traditional meters. As such, these solid state meters also accommodate a 20 amp current transformer output and up to 480 volts for voltage circuits. This is the case even though the electrical burden of such solid state devices is substantially lower than that of the prior electromechanical meters.
Recent solid-state meters typically come in one of two forms. First, transformer-rated meters, as seen by way of example in FIG. 1, typically used in larger business or industrial applications, are configured such that voltages and currents received from a power line transformer 12 (e.g., connected to a power line 11) are electrically connected to different terminals on the electrical interface of a meter 20. Transformer-rated meters 20 can be electrically connected to a power line transformer 12 by one or more voltage transformers 14 (shown as transformers 14a-c, in each of three phases of a power supply). In particular, transformer-rated meters 20 include voltage measurement elements 24 and current measurement elements 26, respectively, connecting the meter 20 to supply lines at voltage transformers 14a-c, and current transformers 16a-c, respectively. A power supply 22 is electrically connected to one of the phases of the power supply, for example the “A” phase as shown in FIG. 1.
Second, self-contained electrical meters 120 (as seen in FIG. 2), typically used in small business applications, are configured such that voltage and current measurement elements 24, 26, respectively, are electrically connected to the same terminals of an electrical interface to the meter. Such meters allow load currents to flow through the meter to a customer facility 18, such that all power delivered to a customer premises passes through the meter. By way of comparison, transformer rated meters are able to be connected to higher voltage and current level services via current transformers and possibly voltage transformers, as they can be connected to service delivered to a customer premises. Both types of meters are designed for connection to multi-phase electrical service connections.
Irrespective of the type of meter used, an electrical meter typically includes a voltage measurement element 24 (an associated current measurement elements 26) associated with two or more of the phases of an electrical service connection (e.g., depending on the type of connection used) and a power supply 22 electrically connected to a phase of the electrical service and used to power the meter and any communication equipment included in the meter. The voltage measurement elements associated with each phase require a reliable, stable electrical connection to reliably detect voltage delivered to the premises (and therefore accurately track energy consumption); as such, other electrical connections within the meter cannot have a significant impact on the input voltage measured at the voltage measurement elements.
In many cases, a power supply 22 included in an electrical meter is a switching power supply designed to operate at a variety of different input voltages, and as such creates non-linear loads. Such loads, as affected by the power supply, can distort a non-“stiff” waveform. In other words, if the input voltage (in particular, the phase to which the power supply is connected) to a meter is susceptible to distortion, it is likely to be distorted by the power supply.
To avoid distortion of an input voltage, a traditional approach for transformer-rated meters has involved use of large-scale transformers, shown as voltage transformers 14a-c in the case of a transformer-rated meter 20, on each of the phases that are input to the meter. These transformers 14a-c provide the “stiff” signal connection for a meter 20, which is otherwise provided in the case of the self-contained meter by the signal received from the power line transformer 12.
Because the burden on each transformer in a transformer-rated meter includes the voltage measurement element (which has a burden of a less than one VA), and on one phase additionally includes a switching power supply (as seen in FIG. 1), voltage transformers are used that have a very high burden rating, such as a 500 VA transformer with an accuracy burden rating of 35 VA. This is required in such systems because it prevents distortion of the input voltage to the voltage measurement element on the phase to which the power supply is connected.
Use of such rated transformers on each phase of an input connection to a meter has drawbacks. For example, such transformers can be heavy and large, and may be difficult to install. In some cases, each transformer can weigh up to about 12-30 pounds. Additionally, such transformers can be expensive when installed on multiple phases of an electrical service, often eclipsing the cost of the meter when considering both the cost of each of the transformers and the cost of installation.
To avoid such costs, some electrical utilities have opted to instead not install protective voltage transformers, but rather allow input to the meter to be directly from an electrical service in the case of both self-contained and transformer-rated meters. Such installations require service personnel to wear protective clothing due to the high voltages and currents involved, and the high risk of arcing that can cause substantial burns or other serious injuries. As such, in even these cases, maintenance costs are high due to the time required for service personnel to put on or take off such protective gear, and also a risk of serious injury is elevated.
For these and other reasons, improvements are desirable.